Wound dressing

ABSTRACT

A method and apparatus are disclosed for dressing a wound. The apparatus comprises a sealing layer comprising at least one orifice, an absorbent layer over the sealing layer, absorbing wound exude and a liquid impermeable, gas permeable filter layer over the absorbent layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Phase of the PCT InternationalApplication No. PCT/GB2008/051089 filed on Nov. 20, 2008, designatingthe United States and published on May 28, 2009 as WO 2009/066105, whichclaims priority to Great Britain Patent Application No. 0722820.8, filedNov. 21, 2007, and Great Britain Patent Application No. 0817040.9, filedSep. 17, 2008. The disclosure of these prior applications isincorporated by reference in their entirety and should be considered apart of this specification.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method and apparatus for dressing awound and a method for manufacturing a wound dressing. In particular,but not exclusively, the present invention relates to a wound dressinguseable during topical negative pressure (TNP) therapy in which thewound dressing itself acts as a waste canister to collect and storewound exudate removed from a wound site.

There is much prior art available relating to the provision of apparatusand methods of use thereof for the application of topical negativepressure (TNP) therapy to wounds together with other therapeuticprocesses intended to enhance the effects of the TNP therapy. Examplesof such prior art include those listed and briefly described below.

TNP therapy assists in the closure and healing of wounds by reducingtissue oedema; encouraging blood flow and granulation of tissue;removing excess exudates and may reduce bacterial load and thus,infection to the wound. Furthermore, TNP therapy permits less outsidedisturbance of the wound and promotes more rapid healing.

In International patent application, WO 2004/037334, apparatus, a wounddressing and a method for aspirating, irrigating and cleansing woundsare described. In very general terms, the application describes thetreatment of a wound by the application of TNP therapy for aspiratingthe wound together with the further provision of additional fluid forirrigating and/or cleansing the wound, which fluid, comprising bothwound exudates and irrigation fluid, is then drawn off by the aspirationmeans and circulated through means for separating the beneficialmaterials therein from deleterious materials. The materials which arebeneficial to wound healing are recirculated through the wound dressingand those materials deleterious to wound healing are discarded to awaste collection bag or vessel.

In International patent application, WO 2005/04670, apparatus, a wounddressing and a method for cleansing a wound using aspiration, irrigationand cleansing wounds are described. Again, in very general terms, theinvention described in this document utilises similar apparatus to thatin WO 2004/037334 with regard to the aspiration, irrigation andcleansing of the wound, however, it further includes the importantadditional step of providing heating means to control the temperature ofthat beneficial material being returned to the wound site/dressing sothat it is at an optimum temperature, for example, to have the mostefficacious therapeutic effect on the wound.

In International patent application, WO 2005/105180, apparatus and amethod for the aspiration, irrigation and/or cleansing of wounds aredescribed. Again, in very general terms, this document describes similarapparatus to the two previously mentioned documents hereinabove but withthe additional step of providing means for the supply and application ofphysiologically active agents to the wound site/dressing to promotewound healing.

The content of the above-noted references is included herein byreference.

However, the above described apparatus and methods are generally onlyapplicable to a patient when hospitalised as the apparatus used iscomplex, needing people having specialist knowledge in how to operateand maintain the apparatus, and also relatively heavy and bulky, notbeing adapted for easy mobility outside of a hospital environment by apatient, for example.

Some patients having relatively less severe wounds which do not requirecontinuous hospitalisation, for example, but whom nevertheless wouldbenefit from the prolonged application of TNP therapy, could be treatedat home or at work subject to the availability of an easily portable andmaintainable TNP therapy apparatus. To this end GB-A-2 307 180 describesa portable TNP therapy unit which may be carried by a patient andclipped to belt or harness. A negative pressure can thus be applied at awound site.

During TNP therapy a portable or non-portable therapy unit generates anegative pressure at a wound site. As fluid, including air as well aswound exudate material is removed from the wound site this must becollected in some manner remote from the wound site. With prior knowntherapy units the collection and storage of wound exudate material istypically carried out by a waste canister connected to a pump unit ofthe therapy unit. The use of a canister, however, can result in thetherapy unit apparatus itself being quite bulky and expensive tomanufacture. Also replacing a canister or a bag in a canister in whichwound exudate is collected can be a time consuming and relativelyunhygienic process.

Prior known therapy units also tend to include a pump which is used togenerate the negative pressure. Such pumps can be costly to manufactureand are relatively heavy.

WO 2007/030601, which is incorporated herein by reference discloses aself-contained wound dressing with a micro pump. The pump for drawingwound fluid into a vacuum zone is included in a wound dressing itself.Nevertheless wound exudate from the dressing can only be removed via acomplex series of steps. The exudate removal process is also prone tocontamination since once an absorbent layer is fully saturated withwound exudate an access door must be opened in the wound dressing sothat the absorbent layer and micro pump can be removed. It will beappreciated that such exudate removal and pump removal can be timeconsuming and can lead to cross contamination between users. A furtherproblem is that the wound dressing is prone to over expansion andrupture.

SUMMARY OF SOME EXEMPLIFYING EMBODIMENTS

It is an aim of the present invention to at least partly mitigate theabove-mentioned problems.

It is an aim of certain embodiments of the present invention to providea method for providing negative pressure at a wound site to aid in woundclosure and healing in which wound exudate drawn from a wound siteduring the therapy is collected and stored in a wound dressing.

It is an aim of certain embodiments of the present invention to providea wound dressing which is able to be placed over a wound site and whichincludes an integrated pump to generate negative pressure at that woundsite. Also the wound dressing can collect any wound exudate.

According to a first aspect of the present invention there is providedapparatus for dressing a wound, comprising:

-   -   a sealing layer comprising at least one orifice;    -   an absorbent layer over the sealing layer for absorbing wound        exudate; and    -   a liquid impermeable, gas permeable filter layer over the        absorbent layer.

According to a second aspect of the present invention there is provideda method of applying topical negative pressure at a wound site,comprising the steps of:

-   -   via a pump element, pumping wound exudate and air from a wound        site, a peripheral region around the wound site being sealed        with a sealing layer of a wound dressing;    -   collecting wound exudate pumped from the wound site, through at        least one orifice in the sealing layer, in an absorbent layer of        the wound dressing; and    -   exhausting gas from the wound dressing through a filter layer        between the absorbent layer and a cover layer extending over the        wound dressing.

Certain embodiments of the present invention provide the advantage thata disposable wound dressing can be fixed over a wound site and cansimultaneously be used to provide negative pressure at the wound siteand collect and store wound exudate. A filter layer of the wounddressing permits air to be continually exhausted from the wound dressingas an integral pump or remote pump generates negative pressure at thewound site. This prevents a risk of over expansion.

Certain embodiments of the present invention provide the advantage thata separate therapy unit is not required to generate negative pressure ata wound site and collect and store any wound exudate. Rather a wounddressing can carry out both a pumping and wound exudate collectingprocess. The wound dressing may then be a one use item which can bedisposed of subsequent to use. This reduces a risk of contamination.

Certain embodiments of the present invention provide the advantage thata wound dressing can be used to collect wound exudate generated during anegative pressure therapy process. A pump remote from the wound dressingcan be connected to the wound dressing and reused whilst the wounddressing itself is used to collect wound exudate and may then bedisposed of after use.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described hereinafter,by way of example only, with reference to the accompanying drawings inwhich:

FIG. 1 illustrates a wound dressing;

FIG. 2 illustrates a top view of a wound dressing;

FIG. 3 illustrates a portion of the wound dressing; and

FIG. 4 illustrates a wound dressing with an external pump.

In the drawings like reference numerals refer to like parts.

DETAILED DESCRIPTION OF SOME EXEMPLIFYING EMBODIMENTS

FIG. 1 illustrates a cross section through a wound dressing 100according to an embodiment of the present invention. A plan view fromabove of the wound dressing 100 is illustrated in FIG. 2 with the lineA-A indicating the location of the cross section shown in FIG. 1. Itwill be understood that FIG. 1 illustrates a generalised schematic viewof an apparatus 100. It will be understood that embodiments of thepresent invention are generally applicable to use in topical negativepressure (TNP) systems. Briefly, negative pressure wound therapy assistsin the closure and healing of many forms of “hard to heal” wounds byreducing tissue oedema; encouraging blood flow and granular tissueformation; removing excess exudate and reducing bacterial load (and thusinfection risk). In addition, the therapy allows for less disturbance ofa wound leading to more rapid healing.

The wound dressing 100 can be located over a wound site to be treated.The dressing 100 forms a sealed cavity over the wound site. Optionallywound packer material can be used within a wound cavity below thedressing. Aptly the packer material can be a gauze or reticulated PUfoam material.

It is envisaged that the negative pressure range for the apparatusembodying the present invention may be between about −50 mmHg and −200mmHg (note that these pressures are relative to normal ambientatmospheric pressure thus, −200 mmHg would be about 560 mmHg inpractical terms). Aptly the pressure range may be between about −75 mmHgand −150 mmHg. Alternatively a pressure range of up to −75 mmHg, up to−80 mmHg or over −80 mmHg can be used. Also aptly a pressure range ofbelow −75 mmHg could be used. Alternatively a pressure range of over−100 mmHg could be used or over −150 mmHg.

As illustrated in FIG. 1 a lower surface 101 of the wound dressing 100is provided by an optional wound contact layer 102. The wound contactlayer 102 can be a polyurethane layer or polyethylene layer or otherflexible layer which is perforated, for example via a hot pin process orin some other way, or otherwise made permeable to liquid and gas. Thewound contact layer has a lower surface 101 and an upper surface 103.The perforations 104 are through holes in the wound contact layer whichenables fluid to flow through the layer. The wound contact layer helpsprevent tissue ingrowth into the other material of the wound dressing.The perforations are small enough to meet this requirement but stillallow fluid through. The wound contact layer also helps hold the wholewound dressing together and acts as a carrier for an optional lower andupper adhesive layer (not shown). For example, a lower pressuresensitive adhesive may be provided on the underside surface 101 of thewound dressing whilst an upper pressure sensitive adhesive layer may beprovided on the upper surface 103 of the wound contact layer. Thepressure sensitive adhesive which may be a silicone or acrylic basedadhesive or other such adhesives may be formed on both sides oroptionally on a selected one or none of the sides of the wound contactlayer. When a lower pressure sensitive adhesive layer is utilised thishelps adhere the wound dressing to the skin around a wound site.

A layer 105 of porous material such as a foam layer or the like islocated above the wound contact layer. This porous layer allowstransmission of fluid including liquid and gas away from a wound siteinto upper layers of the wound dressing. The layer 105 also helpsdistribute pressure generated by a pump, mentioned in more detail below,so that a whole wound site sees an equalised negative pressure.Reticulated foam or a non-woven material which might be natural orsynthetic can be used as the porous material of the porous layer 105.

A gas impermeable sealing layer 106 extends across the width of thewound dressing. The sealing layer which may, for example, be apolyurethane film having a pressure sensitive adhesive on both sides isimpermeable to gas and this layer thus operates to seal a wound cavityover which the wound dressing is placed. In this way an effectivechamber is made beneath the sealing layer and between the sealing layerand a wound site where a negative pressure can be established. Aptlywhilst the sealing layer is gas impermeable the material of the sealinglayer can have a high moisture vapour permeability. For exampleElastollan (Trade name) SP9109 manufactured by BASF. A dotted patternspread acrylic adhesive can optionally be used to help improve moisturevapour permeability. An advantage of using a high moisture vapourpermeability material as the sealing layer 160 is that the fluidhandling capacity of the dressing may be increased significantly by theaction of moisture transpiring through the film and dispersing into theatmosphere. Advantageously, transpiration rates can be easily achievedof the order of 3000 grams/centimeter square/24 hours as a result of thehigh humidity achieved in the dressing and intimate contact of materialachieved during use of the apparatus at a negative pressure of up to 250mmHg below atmospheric pressure.

As illustrated in FIG. 1 a single central orifice is provided in thesealing layer. It will be appreciated that as an alternative the singleorifice 107 may be provided as a plurality of through holes. The orifice107 enables a micro pump 110 with associated valves not shown, which isintegral to the wound dressing, to suck fluid from a region beneath thesealing layer 106 into a region above the sealing layer 106. A pumpinlet is located in a facing relationship to the orifice 107 whilst anoutlet 121 of the pump 110 is arranged to exhaust fluid pumped throughthe pump into a layer 130 of absorbent material. The absorbent materialwhich may be a foam or non-woven natural or synthetic material and whichmay optionally include or be super-absorbent material forms a reservoirfor fluid, particularly liquid, removed from the wound site. Thematerial of the absorbent layer also prevents liquid collected in thewound dressing from flowing in a sloshing manner. The absorbent layer130 also helps distribute fluid throughout the layer via a wickingaction so that fluid is drawn from a region proximate to the exhaustoutlet 121 of the pump into more remote regions so that agglomerationnear the pump exit is avoided.

A filter layer 140 is provided over the absorbent layer 130. A suitablematerial for the filter material of the filter layer 140 is 0.2 micronGore™ expanded PTFE from the MMT range. Larger pore sizes can also beused but these may require a secondary filter layer to ensure fullbioburden containment. As wound fluid contains lipids it is preferable,though not essential, to use an oleophobic filter membrane for example1.0 micron MMT-332 prior to 0.2 micron MMT-323. This prevents the lipidsfrom blocking the hydrophobic filter.

It will be understood that other types of material could be used for thefilter layer. More generally a microporous membrane can be used which isa thin, flat sheet of polymeric material, this contains billions ofmicroscopic pores. Depending upon the membrane chosen these pores canrange in size from 0.01 to more than 10 micrometers. Microporousmembranes are available in both hydrophilic (water filtering) andhydrophobic (water repellent) forms. Aptly the wound dressing 100according to certain embodiments of the present invention usesmicroporous hydrophobic membranes (MHMs). Numerous polymers may beemployed to form MHMs. For example, PTFE, polypropylene, PVDF andacrylic copolymer. All of these optional polymers can be treated inorder to obtain specific surface characteristics that can be bothhydrophobic and oleophobic. As such these will repel liquids with lowsurface tensions such as multi-vitamin infusions, lipids, surfactants,oils and organic solvents.

MHMs block liquids whilst allowing air to flow through the membranes.They are also highly efficient air filters eliminating potentiallyinfectious aerosols and particles. A single piece of MHM is well knownas an option to replace mechanical valves or vents. Incorporation ofMHMs can thus reduce product assembly costs improving profits andcosts/benefit ratio to a patient.

The filter layer 140 thus enables gas to be exhausted upwards throughthe wound dressing. Liquid, particulates and pathogens, however, arecontained in the dressing.

A cover layer 150 covers the absorbent layer of the wound dressing 100.The cover layer which, for example, may be a polyurethane film acts as abacterial barrier and helps hold in liquid to stop fouling. The coverlayer is sealed to the filter layer. For example via adhesive or weldingtechniques. Gas leaving the dressing thus passes through the filterlayer and then the holes 151 in the cover layer. The cover layer alsoprovides integrity for the dressing and is permeable to moisture vapourand gas by virtue of through holes 151 provided in the cover layer, forexample by way of an array of perforations. These help improve thepermeability to moisture vapour and gas through the wound dressing. Inan alternative embodiment the filter layer 140 may instead cover thewhole dressing sealing with the sealing layer 106. A protector layer orreinforcing layer may be included (this may be a non-woven or perforatedfilm). Aptly the cover layer 150 is made from a material having a highmoisture vapour permeability. For example Elastollan (Trade name) SP9109manufactured by BASF. A dotted pattern spread acrylic adhesive canoptionally be used to help improve moisture vapour permeability. Anadvantage of using a high moisture vapour permeability material as thecover layer 160 is that the fluid handling capacity of the dressing maybe increased significantly by the action of moisture transpiring throughthe film and dispersing into the atmosphere. Advantageously,transpiration rates can be easily achieved of the order of 3000grams/centimeter square/24 hours as a result of the high humidityachieved in the dressing and intimate contact of material achievedduring use of the apparatus at a negative pressure of up to 250 mmHgbelow atmospheric pressure.

Turning to FIG. 2 which illustrates a wound dressing 100 in accordancewith an embodiment of the present invention one can see an upper surfaceof the cover layer 150 which extends radially outwardly away from acentre of the dressing into a border region 200 surrounding a centralraised region 201 overlying the absorbent layer 130. FIG. 2 also helpsillustrate the array of perforations 151 formed in the cover layer.

FIG. 3 illustrates an expanded view of the border region 200 of thewound dressing 100 illustrated in FIGS. 1 and 2. As seen, the coverlayer 150 extends over the absorbent layer 130 into an edge region. Herethe cover layer is secured to the sealing layer 106 and the woundcontact layer 102. FIG. 3 also helps illustrate how the perforations 104in the wound contact layer 102 extend around the foam layer 105.

It will be understood that according to embodiments of the presentinvention the wound contact layer is optional. This layer is, if used,porous to water and faces an underlying wound site. A lower porous layer105 such as a reticulated PU foam layer is used to distribute gas andfluid removal such that all areas of a wound are subjected to equalpressure. The sealing layer forms a substantially airtight seal over thewound. Thus as the pump 110 pumps a negative pressure is generated belowthe sealing layer. This negative pressure is thus experienced at thetarget wound site. Fluid including air and wound exudate is drawnthrough the wound contact layer and reticulated foam layer through theorifice 107 and into a fluid inlet 120 at the pump 110. The pumpexhausts the fluid through a fluid exit 121 where the liquid componentis wicked away so as to be collected and stored throughout the absorbentlayer 130. The gaseous components of the exhausted fluid, together withsome moisture vapour, are exhausted through the filter layer and throughthe perforations of the cover layer. The filter layer ensures thatpathogens and unpleasant odours do not leave the reservoir region formedby the absorbent layer.

It will be appreciated by those skilled in the art that rather than havea cover layer overlying the filter layer the cover layer may itself beoverlain by a filter layer. The cover layer may thus be the outermostlayer of the wound dressing or the filter layer may be the outermostlayer of the wound dressing. Further outer layers (not shown) mayoptionally be used so long as they are gas and water vapour permeable.

As still further options the dressing can contain anti-microbial e.g.nanocrystalline silver agents on the wound contact layer and/or silversulphur diazine in the absorbent layer. These may be used separately ortogether. These respectively kill micro-organisms in the wound andmicro-organisms in the absorption matrix. As a still further optionother active components, for example, pain suppressants, such asibuprofen, may be included. Also agents which enhance cell activity,such as growth factors or that inhibit enzymes, such as matrixmetalloproteinase inhibitors, such as tissue inhibitors ofmetalloproteinase (TIMPS) or zinc chelators could be utilised. As astill further option odour trapping elements such as activated carbon,cyclodextrine, zealite or the like may be included in the absorbentlayer or as a still further layer above the filter layer.

FIG. 4 illustrates an alternative embodiment of the present invention inwhich a pump is not provided integral within a wound dressing 400.Rather a connecting tube 401 is connected to an orifice 107 in thesealing layer. The outlet tube 401 is sealed around its outercircumference to the cover layer 150 and filter layer 140. The outletconduit 401 is connected to a remote pump which may be some distancefrom the wound dressing 400 or may be mounted to a border of the wounddressing. An outlet of the remote pump is secured to an inlet tube 402which is sealed about its circumference to a cover layer 150 leading toan opening 403. In this way as a pump is operated to apply a negativepressure to a zone below the sealing layer 106 exudate is drawn throughthe wound contact layer 102 and lower fluid distribution layer 105through the orifice 107 and along the exit tube 401. Fluid, includingair and wound exudate, passes through the pump and is exhausted in aregion proximate to the opening 403 in outlet tube 402. The liquid partof the fluid is wicked away to be collected in the body of the absorbentlayer 130. Air and moisture vapour passes through the filter layer andthrough the apertures in the cover layer into the ambient atmosphere.

It will be understood that according to embodiments of the presentinvention as fluid is drawn from a wound site the absorbent layer mayexpand. This may result in a slight bulge appearing in the upper surfaceof the filter and cover layer. In many respects the bulge can be used asa visible cue to indicate when a wound dressing should be replaced.

As mentioned above with respect to the embodiment shown in FIG. 4 aremote pump may be mounted to a border of the wound dressing in whichcase the tubes 401, 402 may be connected directly to the pump.Subsequent to a single use the wound dressing and pump may thus bediscarded. As an option the tubes may be provided with a click fitconnector or other easy fit connector which can be connected tocorresponding mating connectors joined via corresponding tubes to aremote pump. In this way a remote pump may be reused whilst the wounddressing itself including connecting tubes and connectors is disposableafter a single use.

It will be understood that for embodiments of the present inventionwhich include a pump mounted on the cover layer or on a peripheralborder area of the dressing an integral power source and controlcircuitry can be included. Alternatively the power source can beexternal to the pump and remotely mounted. A remote power source and/orcontrol circuitry improves the disposability of the dressing and permitsbattery recharge if spare batteries are used.

It will also be appreciated that in use the dressing may be used“up-side down”, at an angle or vertical. References to upper and lowerare thus used for explanation purposes only.

It will be appreciated that alternatively the tubes 401, 402 could be asingle dual lumen tube. As a still further alternative the tubes 401,402 could alternatively be a single continuous looped tube, the tubethen passing through pinch rollers for a peristaltic pump.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular aspect, embodiment orexample of the invention are to be understood to be applicable to anyother aspect, embodiment or example described herein unless incompatibletherewith.

What is claimed is:
 1. An apparatus for the treatment of a wound byapplying negative pressure to the wound, the apparatus comprising: awound contact layer comprising a plurality of perforations through thewound contact layer that allows for fluid to pass there through; aporous layer disposed over the wound contact layer configured to allowtransmission of fluid away from a wound site and distribute negativepressure, through the wound contact layer, over the entire portion ofthe wound site being treated; an absorbent material configured to form areservoir for fluid removed from the wound site, the absorbent materialbeing disposed over the porous layer; a cover layer configured to coverthe absorbent material, wherein the absorbent material acts as a wastecontainer to collect and store all liquid removed from the wound sitebelow the cover layer; a hole in the cover layer; a filter positioned atthe hole in the cover layer, wherein the filter is configured to permitsgas to pass through and to prevent the passage of liquid past itself; apump configured to apply negative pressure to the wound site through theperforations in the wound contact layer and through the porous layer andconfigured to draw fluid from the wound site into the absorbentmaterial, wherein the pump is configured to apply negative pressuredirectly to a zone entirely below the absorbent material through a fluidpath that does not contact the absorbent material.
 2. The apparatus ofclaim 1, further comprising a sealing layer disposed between the porouslayer and the absorbent material.
 3. The apparatus of claim 2, whereinthe sealing layer is gas impermeable.
 4. The apparatus of claim 1,wherein the wound contact layer comprises polyurethane and includes anadhesive.
 5. The apparatus of claim 1, wherein the porous layercomprises a synthetic material.
 6. The apparatus of claim 1, wherein thefilter comprises a layer sealed to the cover layer.
 7. The apparatus ofclaim 1, wherein the filter comprises a gas permeable and liquidimpermeable material.
 8. The apparatus of claim 1, further comprising awound packer material configured to be used within a wound cavity belowthe wound contact layer.
 9. The apparatus of claim 1, wherein the coverlayer is permeable to moisture vapor.
 10. The apparatus of claim 1,wherein the cover layer comprises a polyurethane film and an adhesive.11. The apparatus of claim 1, wherein the absorbent material comprises asynthetic superabsorbing material.
 12. The apparatus of claim 1, whereinthe wound contact layer, the porous layer, the absorbent material, thecover layer, and the filter form a wound dressing.
 13. The apparatus ofclaim 1, wherein the cover layer comprises a plurality of holes.
 14. Theapparatus of claim 1, wherein the pump is located under the cover layer.15. The apparatus of claim 1, wherein the pump is a remote pump.
 16. Theapparatus of claim 15, further comprising a conduit sealed to the coverlayer and filter and in fluid communication with the remote pump. 17.The apparatus of claim 15, further comprising a conduit in fluidcommunication with the remote pump connected below the absorbentmaterial to apply negative pressure to a zone entirely below theabsorbent material.
 18. The apparatus of claim 1, wherein the coverlayer is configured to permit moisture vapor permeability whileremaining substantially gas impermeable.
 19. The apparatus of claim 18,wherein the moisture vapor permeability of the dressing permits a fluidtranspiration rate above 3000 grams/cm²/24 hrs.
 20. The apparatus ofclaim 1, wherein the fluid path extends through the entire thickness ofthe absorbent material.
 21. The apparatus of claim 1, wherein the porouslayer disposed over the wound contact layer is configured to distributenegative pressure, through the wound contact layer, equally over theentire portion of the wound site being treated.
 22. The apparatus ofclaim 1, wherein the porous layer is provided directly over theplurality of perforations in the wound contact layer.
 23. The apparatusof claim 1, wherein the porous layer comprises a reticulated material.24. The apparatus of claim 1, wherein the porous layer comprisesreticulated foam.
 25. The apparatus of claim 1, wherein the porous layercomprises a non-woven material.
 26. The apparatus of claim 1, whereinthe wound contact layer comprises an upper surface and a lower surface,and further comprising an adhesive on the lower surface, wherein theadhesive is configured to adhere the wound contact layer to the wound.